Bioengineering bacteria for efficient biofuel production

A report in the current PNAS details a bioengineered bacteria that promises to …

The race to develop alternative fuels is well and truly on. With the full externalities of fossil fuel use now fairly clear, we're going to need a better energy storage medium. The fact that ethanol can be used in internal combustion engines (ICEs) following relatively minor modifications, coupled with our ability to ferment and distill the product from biomass, makes it currently more appealing and practical than hydrogen or electric vehicles. This week's PNAS features an article that details the bioengineering of a bacterium that could lower the cost of bioethanol and made without taking food off peoples' plates.

First-generation biofuels, derived from corn or sugar cane, have taken advantage of our expertise in growing plants with a large amount of simple sugars, which can be readily fermented into ethanol by yeast. However, that same abundance of sugars makes them ideal for food too, and issues surrounding food security, population growth, and biodiversity all make developing newer biofuels worthwhile.

Different approaches are being pursued in order to solve the same problem: how to take energy from the sun and convert it into a hydrocarbon that can be used in ICEs. Those furthest away from commercialization involve bioengineering algae to be one-stop-shops that photosynthesize sunlight and then emit ethanol or hydrocarbons. Other strategies skip the photosynthetic side of things and concentrate on bioengineering bacteria to convert biomass into hydrocarbons; the technology that is poised to have the most immediate impact focuses on cellulosic ethanol.

Cellulose contains sugars that are the products of photosynthesis. Simple sugars and starches, such as those in crop plants, consist of short chains of sugar molecules linked together. Yeast excels at converting these to ethanol (and beer, wine, and spirit drinkers are all very glad of that!).

Cellulose presents a stiffer challenge. Cellulose fibers contain longer polysaccharide chains than those found in starches and surround them with lignin and hemicelluose, which hold the fibers together and provide strength. This makes them tough—tough enough to hold up a tree—but it also makes the sugars within very hard to access.

The study published in PNAS details the engineering of anaerobic, thermophilic bacteria that convert xylose and other nonglucose sugars into ethanol at a high yield. Thermophilic bacteria like heat, and that provides a big boost to the enzymes that break down cellulose. The engineered bacteria happily grew at 50º C, much higher than the maximum temperature tolerated by yeast. At these temperatures, cellulose can be broken down efficiently using less than half the enzymes needed for running the reaction in the presence of yeast.

The engineering step involved eliminating the genes for enzymes that can send the carbon released by the breakdown of sugars down different pathways. With nowhere else to go, all of the sugar wound up converted to ethanol, resulting in a bacterium that's just as efficient as a yeast in ethanol production.

The primary limiting step now is the fact that the bacteria don't tolerate high concentrations of ethanol, which places a ceiling on the efficiency of the process. Selecting for variants that continue to grow as the reaction proceeds should be possible, however, as it's worked for other thermophilic bacteria.

Encouraging developments such as this one are going to be crucial to our efforts to successfully transition away from our dependence upon fossil fuels. The important thing to remember is that no single innovation represents the complete answer in and of itself; a successful energy strategy for transportation will have to make use of every tool at our disposal if we are to have any hope of limiting carbon emissions.